Discussion
In semi-arid alpine regions of the Tibetan Plateau, climatic warming and increased precipitation are expected to cause widespread shrub encroachment into alpine grassland ecosystems (Lu et al., 2022; Wang et al., 2022).Shrubs encroaching will promote plant growth, increase vegetation productivity, and lead to accumulation of soil carbon and nutrients in water-limited alpine grasslands (Pistón et al., 2016; Cui et al., 2023), and, in turn, results in the strengthen of ecosystem functioning (Wang et al., 2023). Consistent with these findings, our study also found that shrub encroachment enhanced ecosystem multifunctionality index of the Tibetan semi-arid alpine steppes (Fig. 4a). Nevertheless, our finding also revealed divergent above- and belowground ecosystem functioning responses to both Leguminosae and Non-Leguminosae shrubs encroachment in semi-arid alpine steppes, with the BEMF index rather than AEMF index increased following shrub encroachment (Fig. 4b-e). Briefly, our results further suggest that shrub encroachment had neutral effects on aboveground ecosystem functions, specifically plant cover, species richness, and below-ground plant biomass (Fig. 2a-e and Fig. 3). However, shrubs encroached into semi-arid alpine steppes generally tend to enhance the belowground ecosystem functions, that is, higher soil nutrients, and soil carbon and nitrogen stocks (Fig. 2f-l and Fig. 3).
The encroaching shrubs of C. spinifera and D. fruticosaare generally drought tolerant and nutrient rich. These encroaching shrubs can create a microhabitat favoring growth of herbaceous plants through buffering of extreme soil temperatures and drought stress and increasing in soil nutrients availability, contributing to the increase of biodiversity and ecosystem functioning (Cui et al., 2023; Zhao et al., 2023). However, species richness, diversity, and productivity of herbaceous plants in the encroached areas also could be lower due to the encroaching shrub species competing for soil nutrients and shading light resources (Zhang et al., 2022). Therefore, the net impacts of shrub encroaching on biodiversity and ecosystem functioning often depend greatly on its facilitation and inhibition effects. In the present study, we found that encroaching of both shrubs tended to increase coverage of the herbaceous plants, but had neutral effects on the diversity and production of herbaceous plants (Fig. 2 and Fig. 3). Thus, the finding indicated that plant diversity and vegetation productivity are an unlikely major mechanism for the shrubs encroaching induced facilitation impacts on ecosystem functioning of the semi-arid alpine steppe.
The availability of soil water and nutrients mainly limited plant growth in alpine steppes (Sun et al., 2020; Cheng et al., 2022). With the encroachment of shrubs, soil organic matter and nutrients gradually accumulate in the soil, creating the ”fertile islands” effects of shrubs (Valencia et al., 2015), which is conducive to the increase of BEMF and EMF. In addition, shrub encroachment could enhance the soil water availability under the shrub canopy by reducing evaporation under the shrub canopy (Iyengar et al., 2017; Liu et al., 2020). It was noted that shrub encroaching induced mitigation of soil water and nutrient deficiencies was the major reason for the abrupt facilitation of ecosystem functioning (Ale et al., 2023; Wang et al., 2023). However, former studies indicated that soil nutrient rather than water availability mainly mediated the encroaching of shrubs caused facilitation impacts on the ecosystem functions of semi-arid alpine grasslands (Cui et al., 2023; Zhao et al., 2023), which is also well reflected by our finding that showed the increased EMF following shrub encroachment (Fig. 4). In the present study, despite the significant relationships between soil water content and species richness of herbaceous plants were showed after encroachment by the tow shrubs, we found that both encroaching shrubs had no significant effects on the SW (Table 3), suggesting that water availability might not be the major reason for the changes of ecosystem functioning of the semi-arid alpine steppes.
Previously have been emphasized that the traits of encroaching shrubs (e.g., Leguminosae or non-Leguminosae species) could affect the degree of their effects on the ecosystem functioning (Eldridge et al., 2011). In semi-arid alpine ecosystems, Leguminosae shrubs, compared with non-Leguminosae shrubs, have great potential to alleviate nutrient limitations and often to facilitate species richness, vegetation productivity, soil microbial activities, and carbon accumulation as well as ecosystem functioning under their canopies (Iyengar et al., 2017; Ale et al., 2023). Our findings showed that the encroaching of both Leguminosae and non-Leguminosae shrubs enhanced ecosystem functions of semi-arid alpine steppes. Nevertheless, the encroachment of both Leguminosae and non-Leguminosae shrubs had significant positive impacts on the belowground ecosystem functioning rather than aboveground ecosystem functioning in semi-arid alpine steppes. This may be because the climate in this region is relatively dry, and shrub encroachment-induced increase in soil nutrients may just slightly promote the growth of herbaceous plants in the semi-arid alpine steppes. In addition, in semi-arid region, most of these herbaceous plants with a shallow root system might be more sensitive to water rather than nutrients availability, and the shallow roots also makes them less advantageous in competing for water and nutrient resources with the deep-rooted Leguminosae shrub (Zhang et al., 2022). Such mechanisms also could explain why there was a neutral response of the AEMF in semi-arid alpine steppes after the encroaching of both shrub species.
Nevertheless, it is critical to note that our study was conducted primarily in the water-limited region on the Tibetan Plateau, and these semi-arid alpine steppes were highly responsive to changes in precipitation and soil water conditions (Zhao et al., 2019). Our findings based on observations highlight that the encroaching of both leguminous and none-leguminous shrubs has the potential to substantially strengthen ecosystem functioning of the semi-arid alpine steppes on the Tibetan Plateau. These findings have important and broadly applicable implications for developing management measures and better maintenance of alpine ecosystems given the vast expanse of alpine steppes across the Tibetan Plateau. Such changes to alpine grasslands ecosystem functioning resulting from shrub encroachment should be adequately considered in predictions of climate changes and anthropogenic activities impact because they could challenge our understanding of how alpine grasslands function.